Suppression of Vortex Induced Vibrations by Fairings

Fairings are nearly-neutrally buoyant devices, which are fitted along the axis of long circular risers to suppress vortex induced vibrations (VIV) and possibly reduce the drag force. Here we study numerically how VIV can be practically eliminated by using free-to-rotate fairings. Since the mass ratio and rotational inertia are both low for the fairings, direct numerical simulations based on standard flow-structure interaction algorithms fail because of the so-called added mass effect. To resolve this problem we introduce fictitious methods and successfully stabilize the simulations. In particular, we investigate the effect of rotational friction $C_f$ on the stabilization effect of the fairings. We found that there exists a critical value for the rotational friction, and when $C_f$ is close to this value, large oscillations and unsymmetrical trajectories can be observed for the riser but for smaller $C_f$ values VIV are suppressed substantially.